Color camera output sampling system



Jan- 26, 1960 w. P. BooTHRoYD ETAL v2,922,837

COLOR CAMERA OUTPT SAMPLING SYSTEM Original Filed Dec. 29, 1950 3Sheets-Sheet 1 oms ma 005 mama/:mr 55:6 amr fn/r 0/7/2' Ja mM, u. l@rra/W70 Jan. 26, 1960 3 Sheets-Sheet 2 Original Filed Dec. 29, 1950 Jan.26, 1960 w. P. BooTHRoYD ET AL 2,922,837

coLoR CAMERA OUTPUT SAMPLTNG SYSTEM Original Filed Dec. 29, 1950 3Sheets-Sheet. 3

a, .WG/ML aum/r mom came/M 7055 /0 #mex/na Pumas 4 L L L L HUUR/76) BY0G/7@ ff). WUI/7745?, JR. v I l KfW/MAMMA wmv-f5@ United States Patent O2,922,837 COLOR'CYAMERA OUTPUT SAMPLING SYSTEM Wilson P. Boothroyd,Huntingdon Valley, and Edgar M.

Creamer, Jr., Philadelphia, Pa., assignors to Philco Corporation,Philadelphia, Pa., a corporation of Penn- Sylvania Original applicationDecember 29, 1950, Serial No. 203,248, now Patent No. 2,769,855, datedNovember 6, 1956. Divided and this application December 30, 1955, SerialNo. 556,749

18 Claims. (Cl. 178-5.4)

This application is a division of application Serial No. 203,248, ledDecember 29, 1950, now U.S. Patent No. 2,769,855, granted November 6,1956.

The present invention relates to a television transmitting system of thetype in which an optical image is analyzed into its primary, orcomponent, colors through the medium of a single camera tube having butone electron scanning beam therein. The invention further relates to animproved form of indexing arrangement for use with such a camera tube toovercome the effects of a nonlinear scanning of the tube target area.

At the present time, all color television systems may be generallyclassified as being either simultaneous or sequential with respect tothe manner in which the image information is conveyed. In the former, aplurality of camera tubes are utilized to develop concurrently aplurality of signal trains, each of which is representative of one ofthe component colors of the object. These individual signal trains arethen transmitted at the same time through separate channels and employedrespectively to modulate the beams of separate image-reproducing tubesat a receiver. The light from each tube may then be focused through anappropriate color lter onto a translucent screen in such a manner thatthe separate component color images are effectively superimposed forviewing by an observer.`

Due principally to the excessive bandwidth requirements of thesimultaneous system, however, it has large- 1y been replaced by thesequential method of transmission. The latter may be further subdividedaccording to the particular manner in which the analysis is carried out,that is-field-by-eld, line-by-line, or dot-by-dot. The first of these,the so-called field-sequential method, is well known, and may includeA aplurality of component-color filters which are successively interposedbetween the photosensitive electrode of the camera tube and the objectwhich is to be televised. This interposition of the filter elements iscarried out mechanically by some such means as a rotating disc or drum.In other words, the field-sequential system employs means whereby anobject eld may be successively scanned in the primary colors, the colorvalues thus being assigned to successive color fields of the image, andthe signals corresponding to these successive elds transmitted through asingle channel.

The so-called line-sequential method of color image representationemploys successively traced image lines which appear at the receiver indiierent colors. It differs therefore from the field-sequential method,in which each separate field is produced in one color only, with thecolors changing from field-to-field through groups of colors which areselected so as additively to produce a polychrome visual image. Thismethod also differs fundamentally from the simultaneous system, in whichseparate image rasters of the additive visual polychrome summation areseparately traced or produced simultaneously in the respective selectedcomponent colors.

The last of the above-mentioned sequential methods,

.when it is reproduced at the receiver.

Vof pulses of video signal energy, with the amplitude of each such pulsebeing determined by the ordinate of the video signal at the preciseinstant at which the pulse is developed. For example, threecomponent-color signals may be respectively developed by three separatecamera tubes, each such color signal being similar to that which isdeveloped inthe simultaneous systemabove referred to. The signal in eachof these video channels' accordingly is continuously present, and issampled in some preferred manner so as to yield a component-color pulsetrain. Then, by means of multiplexing, the three component-color pulsetrains are interleaved into a compositecolor pulse train. While thiscomposite-color pulse train is amplitude modulated, nevertheless theamplitudes of adjacent pulses are entirely independent, inasmuch as theyrepresent separate chromatic aspects of the optical image. After beingfiltered, the composite pulse train is transmitted in any suitablemanner.

One important requisite of any dot-sequential television arrangement aspresently known is that the spacing between the pulses of thecomposite-color pulse train be uniform. In other words, for effectivemultiplexing, it is necessary that the sampling operation be carried outat a constant rate. Any departure of such operation from uniformity willresult in interference between adjacent pulses and correspondingdistortion of the image Thus, successful multiplexing of thecomponent-color signals (without a prohibitive amount of crosstalktherebetween) requires a precise dotor pulse, spacing, both of thepulses in each component-color train as well as of the Ainterleavedpulses in the composite-color train. For that reason, a dot-sequentialtelevision system is normally dependent upon uniform sampling, or, invother words, upon the color information present at the camera tubebeing sampled at a rate which isY unvarying with respect'to time. Anextended discussion of the need for precise time-spacing ofdot-sequential color pulses is given in an article by W. P. Boothroyd inthe December 1949, issue of the publication Electronics on pages 88-92.Furtherl effects resulting from an irregular pulse spacing in a timedivision multiplexing system are brought out in the AIEE Technical Paper#49-25 by W. P. Boothroyd and E. M. Creamer, Jr., dated December 1948.

The above reference to dot-sequential transmission applies toarrangements employing separate camera or pick-up tubes, each of whichis adapted to produce a signal representative of but one component colorof the optical image. In such cases, the output of each camera tube is acontinuous signal, and may be sampled at arbitrarily chosen instants toproduce a representative lvideo pulse train. While it will be readilyapparent that the video output of each camera tube must be available ateach instant of sampling, this is of course the case with a systememploying a plurality of pick-up units, inasmuch as each tube produces avideo output of but a single color.

While a system of the above nature operates to produce an image at thereceiver, it is excessively complex and costly. Accordingly, it isdesirable to eliminate the necessity for employing separate camera tubesfor each component color, and instead utilize a single pick-up tube inthe output of which all of the component-color signals are present. In asystem embodying the present invention, this is preferably accomplishedby providing an optical assembly between the camera tube and the objectto be televised so that each elemental area of the-optical image ispresented to the photosensitive electrode of the camera tube in aplurality of component colors. In a preferred arrangement, 'thesecomponent colors may be Patentedl J an. 26, 1960 y arranged in groups,or units, each of which includes a representation of the value of aparticular elemental area of the image with respect to all the componentcolors. For example, the optical arrangement mentioned may comprise aiilter having a plurality of sets, or groups, of parallel strips, eachstrip in the group passing light therethrough of substantially one coloronly. Thus each fundamental area of the image to be televised is causedto develop separate photo-electric charges on the camera tube mosaicrepresentative of the component colors of such elemental area. As thecathode-ray scanning beam traverses the charged portions of the mosaic,componentcolor signals are successively developed, and appearsequentially at the output of the camera tube. It is of course apparentthat each signal developed by the camera tube as the electron beam scansa charged area of the mosaic represents in effect a sample or indicationof the chromatic characteristics of the optical image with respect tothat particular elemental portion. Hence, the output of the camera tubemay be considered to constitute a series of samples which are capable ofbeing transmitted directly through a communication channel.

When the output of the single camera tube in such a color televisionsystem is transmitted directly, the spacing between the pulses will notnecessarily be constant or uniform unless the cathode-ray scanning beamof the tube is deected in a manner which is exactly linear with respectto time. Any nonlinearities such as might be caused, for example,by.distortions in the shape of the scanning waveform, will result in aseries of output pulses the spacing between which may vary from oneportion of each line-scan to another portion thereof. This may in somecases produce a recognizable image `providing that a scanning operationis present at the receiver which includes the same type of nonlinearityas that present at the transmitter. In practice, however, this isextremely diicult to achieve, and hence for commercial purposes it hasbeen thought necessary to provide some means whereby both the scanningoperations may be made as linear as possible without the employment ofexcessively costly apparatus. Some arrangements have even contemplatedvarious forms of servo mechanisms, in

which the actualrate of deection is compared electriy cally to an idealwave, and any departures from coincidence used to generate a so-callederror voltage which then becomes effective either to speed up or slowdown the scanning beam according to its direction and magnitude ofdeparture from the ideal condition. Moreover, as brought out in theElectronics article above referred to, a direct transmission ofunequally-spaced pulses usually results in an excessive amount ofcrosstalk between the signals of adjacent channels. Consequently, thepresent invention provides means whereby this objectionable eifect ofscanning nonlinearity is overcome.

Another disadvantage in the transmission of camera signals directlythrough a communication channel is that the rate at which the scanningbeam crosses the individual component-color sections of the mosaic maybe entirely distinct from the rate at which it is desired to transmitthe color information to the receiver. For instance, restrictions on thewidth of the channel may require a rate of dot transmission which islower than the rate at which the samples are derived from the cameratube. However, any discrepancy between the sampling rate and thetransmission rate is obviously impossible in systems whereby the derivedpulse train is transmitted directly.

It `has been found possible to employ a camera tube having a sequentialpulse output as above described so that the transmitted signal has apulse rate completely independent of the rate at which the pulses aredeveloped by the camera tube. This can be achieved in accordance withone feature of the present invention by separating the composite camerasignal into three individual signals each of which contains informationas to but one of the component colors. For example, the camera tubeoutput may be gated sequentially to three separate color channels, eachof which then receives only the pulses representative of a particularcolor. If appropriate filters or integrating means are employed, theoutput of each channel will constitute a single substantially continuouscolor-component signal closely approximating that which might beobtained from a simultaneous system employing a plurality of cameratubes `for the various colors and in which due attention is given tolinearity of beam deflection. The three gated, or separated,color-component signals may then be employed in any type of transmissionsystem, whether it be of the simultaneous variety or whether it makesuse of the fleld-, line, or dot-sequential principle. With the separatecomponentcolor signals now available continuously, linal sampling ofthese signals, for example, may be carried out at any desired rate forthe purpose of developing a compositecolor pulse train fordot-sequential transmission inwhich substantially no crosstalk ispresent between the various colors. It will be seen that in such casethe rate of lfinal sampling need bear no particular relationship to theoperation by which the sequential signal output of the camera tube isgated into the separate color signal channels.

The above discussion has assumed that a linear scanning operation ispresent in the single camera tube from which the sequential color outputis obtained, since with a constant, or uniform, gating' apparatusfollowing the camera tube, coincidence of operation is essential inorder to avoid serious distortion. It will be obvious thatrepresentative signals are present in the output of the camera tube onlyat the exact instants when the cathoderay scanning beam is traversingthe charged areas of the mosaic. Hence, gating a signal channel to thecamera tube output at any other instant will not yield the desired colorinformation. lt therefore follows that the cathoderay beam deflectionmust be carried out at the same uniform rate as the gating operation.Even slight departures `from linearity of scan in such a system willcause severe distortion of the reproduced image, and may even result ina completely different color presentation at the receiver from thatwhich is actually represented by the optical image.l This Iis so becausethe signals of on component color may be gated into channels reservedfor signals of another component color if the gating is done at aregular rate and the beam deflection is asynchronous therewith.

In accordance with a still further feature of the present invention,means are provided whereby the objectionable efects of scanningnonlinearities are overcome, this being accomplished without thenecessity of utilizing so-called servo circuits to linearize thedeflecting operation. In the disclosed system, such Vscanningnonlinearities are allowed to remain, and the gating action iscontrolied so that instead of being carried out at a constant rate, itoccurs nonlinearly in synchronism with the nonlinear scanning of thecamera tube mosaic. If the color iilter arrangement previously mentionedis constructed so that some regulating or controlling signal is derivedytherefrom indicative of the progress of the cathode-ray beam acrosssuccessive component-color charge areas of the'mosaic, then thiscontrolling or regulating signal may be employed to modify the normaluniform operation of the gating mechanism. AOne preferred method ofderiving such a controlling or regulating signal is to form the colorlter so that each unit thereof comprises (for a tricolor additivetelevision system utilizing the red, green and blue primary colors) notthree but instead four areas; one of which Vis adapted to produce theregulating signal each time it is traversed by the cathode-ray beam. Forexample, each unit or group of component-color areas may include(sequentially as viewed by the scanning beam) a red area, a blue area, agreen area and an indexing area. The latter area is designed so as toproduce a characteristic signal output pl a from the camera tube that isdifferent from that which would normally be produced by the scanning ofthe red, green and blue areas. Thus, in scanning the camera tube mosaic,signals from the red, green and blue image portions plus an indexingpulse will be sequentially derived. lf the scanning is not carried outat a linear rate, the sucessive interleaved pulses will vary in theirspacing, and similarly the time interval between each indexing pulsewill vary. However, these indexing pulses from the camera tube (afterbeing clipped and shaped if necessary) may be used as control orregulating pulses to trigger the individual color signal separators.Consequently, Vthe composite pulse output from the camera tube may bediverted to the three separate video channels in such time relation thateach signal channel receives samples only of its particularcomponent-color information. lf the nonnal delay period betweensuccessive triggerings of the same separator is made slightly greaterthan the maximum time required for the cathode-ray beam to cross onephotosensitive unit, or group of component-color areas, then the gatingsystem will normally stop until it is next triggered by the passage ofthe beam over the following indexing strip. Thus, the component-colorsignals may be directed into their proper channels even thoughthe'scanning operation of the cathode-ray beam departs considerably froma linear condition.

When the component-color pulse trains which have been separated in theabove manner are respectively applied to low-pass filters each having abandwidth which is slightly less than the average sampling frequency,then the output of each filter may be considered to be essentially thesame output as that derived from one camera tube of a multi-tubesimultaneous color system having substantially linear deflection. If afurther sampler is then utilized which operates at a uniform rate, it ispossible to obtain a dot-sequential color output for transmission whichhas precise dot spacing even though the rate at which the samples areinitially developed in the output of the camera tube is appreciablynonlinear.

While the particular structure for obtaining the regulating orcontrolling signals from the camera tube is not material insofar as theabove system is concerned, nevertheless it is preferred to employ acolor filter construction which produces these indexing signals in aparticularly efficient manner. IIt has been mentioned above that eachgroup of charges on the photosensitive electrode of the camera tubeconsists of four separate areas (for a tricolor television system of thenature described) so that each such unit or group contains red, greenand blue areas plus an indexing charge area. A color lter for developingsuch charges may be constructed by forming channelizing strips in thefilter base between each bundle, or unit, of red, green and blue colorfilter strips. lf the material of which the filter base is made istranslucent, and if the channelizing strips are filled with a diffusingmaterial, then the filter may be edge-lighted so that each lchannelizingstrip will stand out prominently with respect to the illuminationreceived through the color filter strips, and the signal on the cameratube photosensitive surface will be greater in response to the lightreceived from the channelizing strips than it will be for the lightreceived through any one of the color filter areas. Consequently, ascanning of the camera tube mosaic which has been energized by lightfrom such a color filter will provide indexing signals the amplitude ofwhich normally exceeds the amplitude of any of the component-colorsignals. Alternatively, the diffusing material may be omitted, and thechannelizing operation so carried out that at least one surface of eachcut or indentation causes the incident indexing light rays to exceed thecritical angle of refraction and be directed in the general direction ofthe camera tube. -If desired, a very minute lens may be located in thepath of each bundle of refracted light rays to bring them intosubstantially parallel relationship with the filtered image light.

6 One object of the present invention, therefore, is to provide animproved form of color television transmitter of the sequential typeemploying a single camera tube having only one scanning beam therein.

Another object of the present invention is to provide r an improved formof color television transmitting system having a single camera tubedesigned to analyze an optical image in its primary, or component,colors, such tube developing an output signal in which informationrespecting these component colors appear sequentially.

A further object of the invention is to provide for the integration of aperiodic pulse energy so as to yield a continuous wave which may then besampled at equallyspaced time instants to develop a pulse train suitablefor time-multiplexing with a minimum of crosstalk.

A still further object of the present invention is to provide a colortelevision transmitting system of the type described in which thesequentially developed signal is converted into a plurality ofcontinuous component-color signals, and further to provide for theeffective re-sampling of these continuous signals by means operatingindependently of the converting means.

An additional object of the invention is to provide, in a colortelevision transmitting system of the sequential type employing but asingle camera tube, means for sequentially gating the output of thecamera tube to a plurality of component-color signal channels in such amanner that each such channel receives information respecting one of thecomponent colors only.

Other objects and advantages will be apparent from the followingdescription of a preferred form of the invention and from the drawings,in which:

'Figure l is a schematic representation of a color camera employing apreferred form of optical system;

Figure 2 is a face view of the striped color filter assembly of Figure1, showing the relative positions of the indexing strips with respect tothe component-color filter strips;

Figure 3 lis a plan view of the color filter assembly of Figure 2,showing the manner in which the indexing strips are illuminated;

Figure 4 is a waveform of one possible output of the camera of Figure 1when operating linearly as part of a color television transmitter;

Figure 5 illustrates one possible indexing signal derived by clippingthe waveform of Figure 4;

Figure 6 is a block diagram of a complete color television transmitterdesigned in accordance with a preferred embodiment of the presentinvention; and

Figure 7 is a set of idealized waveforms useful in explaining theoperation of the color television transmitter of Figure 6.

Referring now to Figure 1, there is shown a color television camera 8which includes a single pick-up tube 10. This tube 10 as illustrated isof the well-known image Orthicon type, and hence the details thereofneed not be described in the present application. However, the tube willbe understood to include a photocathode 12, on which an image of anobject 14 is focused by means of a lens system 16 through a stripedcolor filter 18. Photocathode 12 is lconnected to the negative terminalof a battery 20 or other source of potential. Illumination falling onphotocathode 12 causes an emission of electrons from the inner surfacethereof, such emission, as is understood in'the art, being Vin the formof an electron image each point of which corresponds in density to thestrength of the illumination on the corresponding point of photocathode12.

The velocity of the electrons thus emitted from the surface ofphotocathode 12 is increased by an accelerating electrode 22 (which isshown as an annular band of metal on the wall of tube 16, but which maybe of any other suitable type, and which is connected to an intermediatepoint on battery l20)v toward a mosaic elec. trode 24. e

Mosaic 24 may, for example, be ofv the double type disclosed by FloryPatent 2,045,984, granted June 30, 1936. The photocathode structure 12may be formed as shown in Patent 2,248,977 issued to Flory et al., onJuly 1,5, 1941` A suitable electron lens (not shown) which may, forexample, be as disclosed in the mentioned Flory et al. Patent 2,248,977,or in Patent 2,189,- 319, issued February 6, 1940, to G. A. Morton, isemployed to focus on the mosaic 24 the electrons emitted from thesurface of photocathode 12.

The electrons impacting the mosaic 24 in turn cause secondary electronsto be released therefrom, these secondary electrons being collected by ascreen 26 which is connected to the positive terminal of battery 20. Therelease of secondary electrons by a particular element, or area, ofmosaic 24 leaves such element with a positive charge, or, in otherwords, with a negative charge deciency. The amount of such deiiciency isdependent upon the density of the electron image at that particularpoint.

The positively charged mosaic 24 is then scanned by means of an electronbeam produced by an electron gun at the opposite end of tube 10, thiselectron gun being of any suitable type which includes a cathode 2S, agrid 30, and an accelerating anode (not shown). The beam deflectingmeans of tube is conventional, and might be magnetic, electrostatic, ora combination thereof. The deflecting electrode system is consequentlyomitted from the drawing for the sake of clarity and simplicity ofillustration.

As the scanning beam travels across the surface of mosaic 24, electronsfrom the beam neutralize the positively charged mosaic elements. Thebeam normally supplies suliicient electrons to make up the negativecharge deficiency of each image point or element. If a particularelement is not positively charged, or if such positive charge is smallenough so that all of the electrons available in the scanning beamduring the instant of passage are not required to make up the negativecharge deficiency on that element, then the remaining electrons in thebeam or, in other words, those not employed to neutralize theelectrostatic charge representing each image point or element, arecaused to return along a path substantially parallel with the scanningbeam toward the end of tube 10 from which they are emitted. Uponarriving at the end of tube 10 containing the electron gun, thesereturned electrons are collected by a signal plate 32 forming a part ofthe tube output circuit. Signal plate 32 may be of any suitable designsuch, for example, as a circular disc having a central aperture thereinthrough which the scanning beam electrons emitted from the cathode 23may pass. The signal from tube 10 is developed across an output resistor34.

It has been stated that an image of the object 14 of Figure l is focusedon the photocathode 12 of camera tube 10 through a striped color lter 18by means of the lens system 16. The latter is designed so that the lightemerging therefrom consists essentially of parallel rays. interposed inthe path of these parallel light rays is the color filter 18, `whichconsists of a plurality of parallel color filter strips 36 arrangedside-by-side in groups upon a translucent base member 38 (see Figures 2and 3). For an additive system of tricolor teievision of the type underdiscussion, the color filter strips 36 may be transparent red,transparent green, and transparent blue, these colors being identifiedin the drawing by the letters R, G and B, respectively. Each coloriilter group or unit thus includes one lter strip of each color as shownin the drawing.

Between each group of color filter strips is a further str-ip, thepurpose of which is to provide an indexing pulse output from the cameratube 10. These indexing strips, identified in the drawing by thereference character l, are preferably formed by channelizing thetranslucent base 38 so as to form a plurality of grooves or indentationstherein. These indentations mayappear in cross-section somewhat as shownin Figure 3--that is, of trapezoidal outline. They are filled with adiffusing material such that when the edge of the filter assembly 1S islighted by a lamp 40 or other source of illumination (Figure l), thediffusing material which fills each channel will stand out as a brightline or bar when viewed from the direction of the camera tube 10. One ofthese bright lines will appear between each set of color lter strips.The intensity of the source of illumination is intended to besufficiently high so that the brightness of the indexing strips I asviewed from the photocathode 12 is greater than the maximum brightnessof any point on the object 14 as seen through any one of the colorfilter strips 36. Consequently, the signal developed across resistor 34in the output of the camera tube 1u will be greater when the cathode-rayscanning beam crosses those particular areas of the mosaic 24 whichcorrespond to the indexing strips, I, than it will be when the beam ison any other portion of the mosaic.

The width of each indexing strip, I, is preferably equal to the width ofeach color filter strip 36, although not necessarily so. Each of theseelements, however, is very narrow, so that one complete unit, or set,consisting of three color filter strips R, G and B and one indexingstrip I, together is equal approximately to the diameter of oneelemental area of the object 14. Then on the photocathode 12 will appeara light image representing the object 14 as seen through the stripedcolor ilter 18, this light image consisting of colored lines separatedby bars of white light. If the resolution of the camera tube 10 is suchthat the individual color lines on the mosaic 24 can be resolved, thenthe output of the camera tube 10 as developed across resistor 34 willconsist of a sequence of voltages, as shown in Figure 4, which representsuccessive cycles of color information, and which are equallytime-spaced so long as the sweep rate of the cathode-ray beam islineal'. it might be said that each elemental area of the object 14 nowcontains a white reference signal, followed by a red sample, then agreen sample, and then a blue sample voltage. The next picture elementcontains exactly the same information, and so on across the line. Sincea televised object seldom contains any completely white portions, it isnormally possible to separate the indexing pulses from :the camera tubeoutput by simple amplitude selection (above the clipping level of Figure4) so that an indexing signal such Ias shown in Figure 5 is derived.

The above-described color television camera is disclosed and claimed inthe aforementioned parent application of which this application is adivision.

A color television transmitter according to the present invention,preferably utilizing the color camera of Figure l, is illustrated inFigure 6. lt has been stated above that the output of the camera may bean equally time-spaced wave such as shown in Figures 4--that is, aseries of pulses developed by the sequential scanning of the colorfilter strips and indexing strips by the cathode-ray scanning beam. Theamplitude-modulated signal of Figure 4 has its maximum values at theinstants when the beam scans the mosaic locations corresponding to thelighted indexing portions of the color filter 1S. As an illustrativeexample, for a 63.4 microsecond sweep time, and with an active intervalof 82% as with present standards, a 2.67 megacycle interlaced dottingsignal, will contain approximately 277 groups of interleaved three-colorand indexing signals for each line scan of the mosaic electrode 24.

The electron'scanning beam of the camera tube lil is deilected in anyconventional manner, and the present invention makes provision for anynonlinearities which may arise in the sweep rate of such beam and whichmay, for example, yield a camera tube output wave such as shown to anexaggerated degree in Figure 7 (a). ln practical applications avariation of several percent may be encountered, and while such anonlinearity would normally be unacceptable as resulting in prohibitivedistortion in a dot-multiplexing system, the present invention utilizesthe indexing pulses present in the output of the camera tube to controla gating operation in a manner now to be described.

Referring to Figure 6, it will be seen that the color camera 8 of Figurel has its output applied in parallel to a red color signal separator 42,a green color signal separator 44,- a blue color signal separator 46,and a clipper and shaper 48. In other words, the composite-color andindexing Wave shown in Figures 4 and 7(11) is present at the input toeach of these four units. It is now desired to gate such wave througheach of the separators 42, 44 and 46 in such a fashion that the outputof each contains information as to its particular component color only.This is accomplished by means of the indexing pulses derived from thewave and shown in Figures 5 and 7(1)) l The clipper and Shaper 48 actsto clip olf the peaks of the indexing pulses above the clipping level inFigures 4 and 7(a) and to shape .and amplify such pulses so that theoutput of the unit 48 consists essentially of a sine wave at indexingpulse frequency. This sine wave is now used as a time base for gatingthe color-component portion of the camera output signal. However, thisindexing wave is preferably first passed through a two-to-one frequencydivider 50 (when horizontal dot-interlacing is employed) and thenapplied to a phase shifter 52 which acts to provide three output wavesfor each input cycle, the three output Waves being spaced apart by 90electrically. The timing of the phase shifter 52 is so set that for eachindexing pulse from the color camera, the three triggering impulsesproduced by the shifter 52 act at 90, 180 and 270 intervals to open thecolor signal separators 42, 44 and 46 respectively at the preciseinstants when the red, green and blue color-component signals appear inthe output of the camera tube. In this manner no gating will occur untilsuch time as an indexing pulse has been applied to the clipper andshaper 48. Since the sine wave output of the divider 50 isfrequency-modulated, any filtering action of the clipper and shaper 48must be such as to permit modulation of the basic indexing rate inaccordance with the sweep nonlinearity.

From the color signal separators 42, 44 and 46 there has now beenobtained three signals (shown in curves (c), (d) and (e) of Figure 7)which have been derived by generally uniform gating at a frequency whichis only approximately correct-that is, the signals have been in effectfrequency-modulated by the nonlinearity of the Y camera sweep. If noweach of these three color-component pulse trains is applied to a filterhaving a passband which is slightly less than the average samplingfrequency in each component-color channel, then the output of each suchfilter will be a substantially continuous signal, approximately as shownin curves (f), (g) and (h) of Figure 7, similar to the signal which isderived in each color channel of a multi-tube simultaneous system inwhich due attention is given to sweep linearity and registration. Forexample, if the sampling rate is 2.67 megacycles per second, then thepassband of each filter should be slightly under this value. Thus inFigure 6 the output of the r'ed color separator 42 is passed through afilter 54 having a passband from zero to approximately 2.5 megacycles,y

while the output of the green and blue color separators 44 and 46 aresimilarly passed through two filters 56 and 58, respectively, having thesame cut-ofi frequency.

The action of the filters 54, 56 and 58 thus is an integrating one, theapplied pulses being smoothed or stretched out so as to leave no gapstherebetween. A somewhat similar result may be achieved by utilizing aclamping circuit in place of each filter unit 54, 56 and 58, or byemploying any other suitable form of integrating device.

Since the respective outputs of the filters 54, 56 and 58 are separatecomponent-color video signals, they may be applied to any form of colortransmitter either of the vquency such as 12.7575 megacycles.

simultaneous or sequential type. However, in accordance with oneembodiment of the present invention, the sequentially-producedcomponent-color signals are now re-sampled for transmission as dots orpulses of color information.

The apparatus for carrying out one form of re-sampling or re-dottingincludes three modulating units, a red modulator 60 connected to theoutput of the filter 54, a green modulator 62 receiving the output ofthefilter 56, and a blue modulator 64 connected to the output of thefilter 58. In order that these three component-color signals besuccessively sampled at equally-spaced time instants, a gating, orsampling wave having a frequency of 3.189375 megacycles (hereinafterdesignated for convenience as 3.19 megacycles) is developed by afrequency divider 66 which is connected to the output of a crystal`oscillator 68 operating at some suitable multiple fre- The 3.19megacycle sampling wave from the frequency divider 28 passes throughaphase shifting unit 70 which acts to provide three output waves each ofwhich is displaced in phase by approximately 120. The phase shifter 70may operate in a manner somewhat similar to the phase-shifting unit 52mentioned above. The three out-of-phase voltage variations from theshifter 70 are respectively applied to the modulators 60, 62 and 64 soas to activate the latter in timed sequence and yield the three pulsetrains shown in curves (i) (j) and (k) of Figure 7. The outputs ofmodulators are combined to form a composite pulse train (curve (1)) forapplication to a low-pass filter 72, the pulses of this composite trainoccurring at a rate of approximately 9.567 megacycles per second.

The filter 72 is designed to have a passband from zero frequency toapproximately four megacycles, with a sharp cut-off at the latter point.The output of the filter 72 is applied to an equalizer unit 74 whichacts as a phase and amplitude corrector. The two units 72 and 74 incombination have the property of passing sampled information at a rateof 8 million samples per second without appreciable crosstalk betweenadjacent pulses. Amplitude versus frequency curves for these units areset forth in the drawing, although the response of each unit is ofcourse chosen in view of the particular operating characteristics of thesystem. The output of the -equalizer 74 is then applied to modulate astandard television transmitter 76 which is connected to antenna 78. Itwill be understood that, if desired, the equalizer unit 74 may be of thetype shown in a copending United States patent application of W. P.Boothroyd, filed January 14, 1949, as Serial No. 70,951.

The sampling circuit which includes the modulators 60, 62 and 64 mustoperate in synchronism with a corresponding sampling device at thereceiver in order to avoid distortion of the reproduced image. Means forcoordinating the operation of the two samplers is fully set forth in acopending United States patent application of R. C. Moore, Serial No.175,438, filed July 22, 1950, and it will only be statedhere'in thatthis means includes a sync and burst injector unit 80 and a burst Shaper82, each of which is connected to a synchronizing generator 84. Thelatter operates to supply horizontal and vertical blanking pulses in theusual manner to the color camera 8, such as `by* application to thescreen 26 of the camera tube 10 in Figure 1. The burst shaper 82 acts asa gate which is opened by the synchronizing generator 84 during aportion of each horizontal blanking interval to permit passagetherethrough of the 3.19 megacycle wave developed by the frequencydivider 66. By this mode of operation, a burst of high-frequency energyis applied through the injector 80 to the input of filter 72 duringhorizontal blanking when no video signal is being received from thecolor camera. As brought out in a further copending United States patentapplication, Serial No. 139,928, tiled January 21, 1950, yby W. P.Boothroyd and E. M. Creamer', Jr., this periodic burst of higlbfrequency energy is kutilized for synchronizing the operation of thereceiver sampling apparatus. ln addition, a vertical timing pulse is.obtained from the sync generator 84 and applied to the burst shaper S2over a connection 86 so that the 3.19 megacycle output of the frequencydivider 66 is prevented from entering the video circuit during thevertical equalizing and blanlting pulse periods. Otherwise, the burstenergy may adversely affect the vertical synchronizing process at thereceiver.

In application Serial No. 139,928, it was stated that the phase of the3.19 megacycle wave may be reversed at a 15-cycle rate at thetransmitter (during vertical rctrace) in order to reverse the phase ofthe receiver sampler and hence obtain horizontal interlacng of the dotinformation. In other Words, by such an expedient the matrix of pointsat which information is extracted from the original scene and reproducedon the display cathode-ray tube may be shifted horizontally, by anamount equal to one-half the distance between the centers of adjacentdots in the picture produced during one field, within every othervertical blanking interval. ln application Serial No. 175,438 the phaseof the 3.19 megacycle wave as applied to `the filter 72 through theburst shaper S remains constant, and the synchronizing pulse output ofthe generator 84 is caused to vary periodically.

lt has been stated above that the color camera S is supplied withblanking pulses directly from the sync generator 84, and also that thelatter acts to control the burst Shaper 82, opening `and closing suchunit so that the injection of the high-frequency 3.19 megacycle energyinto the video circuit occurs preferably during that portion of thehorizontal blanking interval which follows the horizontal synchronizingpulse itself. Furthermore, the mixed sync pulses are applied directly tothe filter 72 from the generator 8d through the sync and burst injector8i). These mixed sync pulses also control in part the gating operationof the burst Shaper S2.

It is desirable that the operation of the sync generator 84 be locked inwith the operation of the horizontal oscillator 68 which provides thehigh-frequency dotting wave through the frequency divider 66. This isbrought about by feeding a portion of the output of the oscillator 68 toa further divider 88 which reduces the frequency of the 12.7575megacycle wave to a value of 94.5 kilocycles.`

The generator S4 is connected to the divider 8S through a gate 90, sothat, when the latter is open, both the frequency and phase of the syncpulses from the gen erator 84 are the same as that of the wave fromdivider 88.

In order to bring about horizontal dot-interlacing without periodicallyshifting the phase of the 3.19 megacycle wave output of the divider 66,it is necessary that the time relation between the sampling wave and thesync pulses be periodically varied through control `of the syncgenerator 84. If the timing of the sync pulses produced by the syncgenerator is shifted at a 30-cycle rate, then the desired relationshipbetween the sync pulses and the sampling Wave will be established. Forthis purpose, there is provided the keyer 92.

This keyer unit 92, which actually is a '3U-cycle square wave generatorcoordinated with the sync generator 84 by means of vertical timingpulses obtained from the latter over a connection 94, is designed toproduce two 180 out-of-phase square waves which change in polarity everyone-sixtieth of a second. Connected to the output of the frequencydivider 8S is a delay circuit 96 which acts to provide a delay intervalequal to onequarter the period of the dot frequency. For a system suchas is described above, this period of delay amounts to approximately342,76 microsecond. The wave output of the delay circuit 96 is thenapplied to the sync generator .84 through a gate 98. Each sync pulsefrom the generator 84 thus has the same time relation with respect to aparticular pulse received through gate 98 from the delay circuit 96 thatit has with respect to this same pulse vwhen the latter is receiveddirectly from divider 8S through gate 90.

As shown in the drawing, the keyer 92 operates alternately to open andclose the gates and 98, so that the timing of the 94.5 kilocycle controlwave from the divider 8S is shifted every one-sixtieth of a secondcorrespondingly to change the time position of the sync pulses in theoutput of the generator 84 relative to that of the sampling wave fromdivider 66. This is equivalent to advancing the positions of the dots inthe image reproduced at the receiver by one-quarter of the horizontaldot spacing in alternate fields, and hence improved interlacing isaccomplished without the necessity of changing the phase of the samplingapparatus either at the transmitter or at the receiver.

in place of the elements 48, 50 and 52 of Figure 6, it is possible tosubstitute other gating devices which will perform satisfactorily forthe purpose of the present disclosure. For example, the continuouslydriven gating apparatus of the drawing may be replaced by a triggeredgate which operates in response to the reception of an indexing pulsefrom the color camera 8 sequentially to open the three gates 42, 44 and46 at the proper time instants. Such a triggered gate, for example, maycomprise a chain-type impulse generator the normal period of operationof which is preferably set slightly longer Vthan the maximum timerequired during each sweep period for the cathode-ray scanning beam tocross successive indexing strips. In this manner the red, green and bluecolor signals will be separated into their respective channels, with thegating always occurring while the beam is centered on a proper mosaicarea.

We claim:

1. ln a color television transmitter: a camera tube having a targetelectrode; means for developing upon said target electrode anelectrostatic charge image descriptive of an optical multi-color imageto be televised, each elemental area of said optical image beingrepresented by a corresponding surface portion of said target electrode,the latter being designed for successive traversal by the cathode-raybeam of said camera tube during each line-scanning operation, each ofthe said surface portions of said target electrode including at leasttwo regions respectively representative of different chromatic aspectsof that elemental area of the optical image covered by that particularsurface portion and in addition at least one region designed to producean indexing signal pulse when impinged by said cathode-ray beam, wherebythe signal output of said camera tube during each line-scanningoperation will be a sequence of sets of Acomponent-color pulses andindexing pulses; a plurality of output channels equal in number to thenumber of color-representative regions in each target surface portion,each output channel being intended for componentcolor signal pulsesderived from similar color-representative regions of said target; andgating means, controlled by the said indexing signals, for directing thecomponentcolor signal pulses sequentially appearing in the output ofsaid camera tube into the respective output channels.

2. A color television transmitter according to claim 1, furtherincluding a plurality of filters to which the sequentially-appearingcomponent-color signal pulses in the said output channels arerespectively applied, the passband of each such filter being less thanthe recurrence frequency of its associated component-color signalpulses, whereby a plurality of substantially continuous filter outputwaves are obtained.

3. A color television transmitter according to claim 2, furthercomprising a sampling device in the output of each of said filters, eachsuch sampling device operating at a constant rate so as to yield aseries of componentcolor pulses the time-spacing of which is uniformregardless of any variation which may occur in the time-spacing of thesequentially-appearing componentcolor pulses at the input to itsassociated filter.

4. In a color television transmitter: a camera tube having a targetelectrode; means for developing upon said target electrode anelectrostatic charge image indicative of an optical multi-color image tobe televised, said target electrode being made up of a plurality of setsof elongated color-representative areas lying substantially transverseto the line-scanning action of the cathode-ray beam of said tube, eachsuch set of elongated target areas including one area representative ofeach one of the primary colors of that elemental region of the opticalimage covered thereby, and, in addition, one area designed to yield anindexing pulse when impinged 'by said cathode-ray beam, whereby thesignal output of said camera tube during each line-scanning operationwill be a sequence of sets of primary-color signal pulses and indexingsignal pulses; a circuit for separating the indexing signal pulses inthe output of said camera tube from the remaining primary-color signalpulses; a plurality of color channels each intended to receive signalsof but one primary color; and gating means, controlled by the saidseparated indexing signal pulses, for directing thesequentially-appearing primarycolor signal pulses in the output of saidcamera tube into the respective color channels regardless of whether ornot the said cathode-ray beam line-scanning action takes place at aconstant rate with respect to time.

5. A color television transmitter according to claim 4, furtherincluding a plurality of filters respectively receiving thesequentially-appearing primary-color signal pulses in the said colorchannels, the passband of each such lter being less than the recurrencefrequency vof the particular primary-color signal pulses receivedthereby, as a result of which the output of each lter comprises asubstantially continuous primary-color signal wave.

6. A color television transmitter according to claim 4, in which thesaid gating means is normally inoperative until triggered by thereception of one of the said indexing signal pulses, whereupon saidgating means acts to direct into their respective color channels theprimarycolor signal pulses derived from the scanning of one only of thesaid sets of elongated target areas, following which the said gatingmeans again becomes inoperative until triggered by the next succeedingindexing signal pulse.

7. In a color television system, and in combination, a screen having acyclically repeated plurality of sets of parallel strips, each setcomprising strips corresponding in number and in predetermined order toa number of component colors to be reproduced, means for scanning thesaid screen by moving an electron beam transversely in a singledirection across all of the strips to scan by lines, means responsive tothe electron beam for generating a signal in timed relation to thescanning thereby of the strips, the last said means comprising means forclipping video signals to produce a signal corresponding to video signalabove a predetermined magnitude, such magnitude being greater than themaximum video signal to be transmitted, and means controlled by the saidsignal for generating a plurality of video switching signalscorresponding in number to the number of component colors, in frequencyto the scanning of the sets of strips by the electron beam and in phaseto the spacing of strips within a set.

8. Color television apparatus comprising an image pickup device forgenerating a signal train comprising signal elements intercalated-according to a color sequence and representative of diierent colorcomponents of elements of the image, means for producing signal elementsrecurring in a predetermined sequence, in said signal train and connedto a separate amplitude range from signal elements representing saiddiierent color components, -a plurality of signal channels with one foreach to said scanning,

14 of said color components, a gate leading to each signal channel,means for applying said signal train to said gates, and means responsiveto said recurring signal elements for opening said gates in successionto admit the signal elements representative of said different colorcomponents to the corresponding signal channels.

9. Color television apparatus comprising an image pickup device forgenerating a signal train comprising signal elements intercalatedaccording to a color sequence and representative of dilerent colorcomponents of elements of the image, means `for producing signalelements recurring in a predetermined sequence in said signal train andconfined to a separate amplitude range from signal elements representingsaid different color components, a plurality of signal channels with onefor each of said color components, a gate leading to each signalchannel, means for applying said signal train to said gates, and agating pulse generator synchronized by said recurring signal elements,said gates being responsive to pulses from said pulse generator to causesaid gates to open in succession to admit the signal elementsrepresentative of different color components to the corresonding signalchannels.

10. A color television pickup tube system comprising in combination, apickup tube including an electron target structure, a plurality ofinterleaved sets of optical lter strips having respectively dierentcomponent color pass bands, at least one set of reference strips-associated with said filter strips, means for generating an electronbeam, means for. scanning said target structure with said electron beam,means including said filter strips for delveloping a composite signalcontaining information relative to the different color components of asubject image in response to said scanning, means for deriving areference signal in relation to said reference strips in response andmeans for utilizing said reference signal to derive a component colorsignal from said composite signal.

11.` A pickup tube system in accordance with claim l0 wherein saidcomposite signal is derived from the return beam of said pickup tube.

l2. A colortelevision pickup tube system comprising, in combination, animage pickup tube including a target, a plurality of interleaved sets ofoptical filter strips of respectively different component color responsepositioned to intercept light directed to said pickup tube from asubject to be televised, at least one set of reference strips associatedwith said optical iilter strips, means for developing an electron beamin said pickup tube, deiecting means for causing said beam to traversesaid target, means for deriving from said pickup tube in response tosaid traversal a composite signal including information concerning eachof the respective color components of the subject, a plurality `ofmodulators each having input and output circuits, means for impressingsaid composite signal upon the input circuits of all of said modulators,means for deriving reference signals in relation to said referencestrips in response to said traversal, and means for impressing each ofthe respective reference signals upon the input circuit of arespectively diterent one of said modulators whereby each of saidmodulators provides an output signal containing information concerningbut a respective one of said color components.

13. A color television pickup system comprising, in combination, apickup tube including an electron target structure; at least one set ofreference strips positionally related to target areas representative oflight .of -a cornponent color from a subject to be televised; saidpickup tube including means for generating an electron beam, means forcausing said beam to trace a scanning raster upon said target structure,and means for collecting the electrons of said beam returned from saidtarget structure throughout said raster tracing; means for derivingreference signals in relation to said reference strips in response tosaid raster tracing; a plurality of modulators;

means vfor applying the return beam signal appearing at said collectingmeans to each of said plurality of modulators; and means foradditionally applying to each of said plurality of modulators arespectively different one o-f said reference signals.

14. A color television camera comprising in combination a color imagepickup tube including an electron target structure, means for developingan electron beam, and means for causing said beam to trace a scanningraster upon said target structure; a plurality of interleaved sets ofoptical filter strips having respectively different components colorpassbands and through which light directed to said pickup tube from asubject to be televised passes; at least one set of reference stripspositionally related to said interleaved filter strip sets; means forderiving from said pickup tube through the tracing of said raster acomposite signal which includes respective components representative ofdifferent component color aspects of said subject; means for derivingreference signals in relation to said reference strips throughout saidraster tracing; and means for utilizing each of said respective ret'-erence signals to separate a respectively `different one of saidcomponents from said composite signal.

15. In a color television system, a pickup tube of the type whichproduces an image-representative signal by scanning action of anelectron beam, a plurality of optical iilter elements arranged so as tocause said signal to comprise signal components successively andrepetitively representative of a number of diierent chromatic aspects ofelemental areas of the image, means including a plurality of referenceelementsl associated with said iilter elements for causing production ofreference signal components by the scanning action of said beam, aplurality of signal channels equal in number to the number of differentchromatic aspects represented by the first-mentioned signal components,means for supplying said imagerepresentative signal to said channels,and means controlled by said reference signal components for controllingsaid signal channels so to effect transfer of the differentchromaticity-representative signal components through the differentsignal channels.

16. A color television image pickup system according to claim l5,wherein said lter elements and said reference elements are in the formof strips.

17. In a color television system, a camera tube for producing a signalvoltage indicative of the magnitudes of the color components of thecolor content of an object,

Said tube .comprising photosensitive means including an image surf-acescanned by an electron beam in successive line scannings,y means fordirecting light from saidV object onto said photosensitive means, colorfilter means for rendering successive elemental areas of said surfaceresponsive to light of different colors derived fromsaid object, wherebythe scanning of said surface produces image signal components which aresuccessively and repetitively representative of diiferent chromaticaspects of elemental areas of said object, means including referenceelements` associated with said color filter means for producingreference signal components in response to the scanning of said surface,la plurality of signal channels equal in number to the number ofdifferent chromatic aspects represented by the first-mentioned signalcomponents, means for supplying said first-mentioned signal componentsto said channels, and means controlled by said reference signalcomponents for controlling said signal channels so as to etfect transferof the different chromaticity-representative signal components throughthe different channels.

1S. in a color television transmitter, means for producing pulses whichare successively and repetitively representative of a number kofdifferent chromatic aspects of elemental areas of an object beingtelevised, means for producing indexing signal pulses which bear apredetermined time relation to said chromaticity-representative pulses,a plurality of signal channels equal in number to the number ofchromatic aspects represented by said first signal pulses, meanscontrolled by said indexing signal pulses for controlling said signalchannels so as to eiect transfer of the differentchromaticity-representative pulses through the different signalchannels, a iilter in each of said channels having a pass band less thanthe recurrence frequency of the chromaticity-representative pulses,whereby a substantially continuous wave is produced in the output ofeach filter, and means for sampling the output of each filter at aconstant rate so as to yield a series .of chromaticity-representativepulses the timespacing of which is uniform regardless of any variationwhich may occur in the time-spacing of the pulses at the input of thefilter.

References Cited in the le of this patent UNTED STATES PATENTS 2,630,485Heikes et al. Mar. 3, 1953

